Fix circular reference recursion in JSON serialization

Dataclass models have parent back-references (TrackLayout→Circuit→
Locality→Country) causing infinite recursion in dataclasses.asdict().
Add to_dict() methods that exclude parent refs, use them in API endpoints.

models/track_layout.py

@@ -27,6 +27,13 @@ class TrackLayout:
             circuit=circuit
         )
 
+    def to_dict(self) -> dict:
+        return {
+            "slug": self.slug,
+            "description": self.description,
+            "imageUrl": self.imageUrl,
+        }
+
     @property
     def coordinates(self) -> list[tuple[float, float]]:
         return self._geo_data.features[0].geometry.coordinates